Like bimonthly and biennially, but unlike biannually, in the adjective and adverb senses, the prefix bi- applies to the week rather than to the events that occur, therefore giving the sense of “every two weeks”. Since the term is also used in the UK to mean “twice a week”, it is ambiguous there. In British English, for the sake of clarity, it is advisable to use one of the synonyms instead. Furthermore, in some English speaking regions, the term fortnightly is also commonly used to describe an event happening once every two weeks.

Unlike with bimonthly and biannually, in the adjective and adverb senses, the prefix bi- applies to the week rather than to the events that occur, therefore giving the sense of “every two weeks”. Since the term is also used in the UK to mean “twice a week”, it is ambiguous there. For the sake of clarity, it is advisable to use one of the synonyms instead.